THE PRESENCE OF METASTABLE HNO3 H2O SOLID-PHASES IN THE STRATOSPHERE INFERRED FROM ER-2 DATA/

We present data to show that at least some of the observed polar strat ospheric cloud (PSC) particles sampled by the ER 2 on January 20, 1989 , during the Airborne Arctic Stratospheric Expedition (AASE 1) were co mposed of a water-rich HNO3/H2O solid phase. The PSC water content der ived from the ER 2 data on this day is larger than that of nitric acid trihydrate (NAT), nitric acid dihydrate (NAD) or an aqueous ternary s olution of H2SO4/HNO3/H2O. Here, we suggest that these particles were composed of a water-rich metastable HNO3/H2O solid phase and refer to such clouds as Type Ic PSCs, which are different from Type la (crystal line NAT or NAD particles) or Type Ib (aqueous ternary solution drople ts) PSCs. Type 1c PSCs could either be crystalline (a higher hydrate o f HNO3 and H2O) or amorphous, and the data analysis presented here can not distinguish between these different solid phases. The observed PSC on this day could have been a mixture of water-rich HNO3-containing s olid particles with liquid ternary droplets and/or NAT (NAD) aerosols. However a mixture of cloud particles composed of ternary solution dro plets and NAT (NAD) aerosols is inconsistent with the data. A surface adsorption/reaction process occurring on frozen sulfate surfaces is su ggested as a formation mechanism for Type Ic PSCs in the stratosphere. A possible mechanism for the formation of large HNO3-containing aeros ols (NAT or NAD) starting with Type Ic PSCs is discussed.